Low energy electrical arcs are used as ignition sources in various industrial and academic test systems, such as in systems that test flammability properties of materials. For example, an electrical arc may be used as an ignition source to measure the ignitability of a gas or a gaseous mixture, such as a fuel-air mixture. Generally, the electrical arc is generated by a sparking device that includes electrodes that are spaced apart from one another. A material to be tested is disposed in a region between the electrodes, and a voltage is applied to the electrodes. The electrical arc is generated between the electrodes when the voltage across the electrodes exceeds a breakdown voltage of the material to be tested.
For some tests, the energy level of the electrical arc must be carefully controlled to fall within a specified range. For example, a test procedure that the Federal Aviation Administration (FAA) requires for testing the ignitability of fuel-air mixtures used for aircraft calls for the use of an electrical arc of 200 microjoules plus 0 microjoules minus 20 microjoules. Due to the short duration and very low energy levels of such electrical arcs, direct measurement of the energy level of the electrical arcs is not generally possible. This has led academic and industrial testing to focus on the stored potential electrical energy that may contribute to an electrical arc as the quantity to which to calibrate flammable gas ignitability rather than the actual energy dissipated in the electrical arc channel itself.
In many test systems, the energy storage system uses a capacitor to store the energy. At such low energy levels (e.g., in the 100 microjoule range) and with such tight tolerances (e.g., several microjoules), minor changes in circuitry coupled to the sparking device can result in significant changes in the energy of the electrical arc. For example, the breakdown voltage of a fuel-air mixture may be on the order of kilovolts. To provide a voltage across electrodes of the sparking device on the order of kilovolts with a total energy storage on the order of microjoules requires a capacitance on the order of picofarads. At such small capacitances, the capacitance of the entire circuit coupled to the sparking device must be taken into account since changes as minor as shifting the relative positions of two wires can cause changes in the capacitance of the circuit that effect the test. As a result, testing systems that use low energy, high precision electrical arcs are notoriously difficult to use.